Refrigeration systems typically include a condenser, an evaporator, an expansion device, and a compressor driven by an electric motor. The entire system is usually hermetically sealed to prevent refrigerant leakage.
In many cases, the motor has six winding leads that can be connected in different configurations for connection to either a high or low voltage three-phase power source. For a low voltage hookup, the winding leads are grouped into three pairs, and each pair is connected to a different phase of the three-phase power source. In a high voltage application, only three of the six winding leads are directly connected to the power source. The remaining three leads are tied together and mounted at a location where they will not arc to ground nor to any of the other three leads.
Mounting the three common leads to a fourth terminal on the motor's terminal board would be one possible location. Such a location, however, is often impractical if the motor housing is pressurized with refrigerant and the supply voltage is relatively high, such as 1,000 volts or more. The location is impractical under such operating conditions, because the size of terminal board is usually too small to include four terminals.
The terminal board is attached to the motor housing and includes three feedthroughs that couple the motor windings to the three phases of the power source. The terminal board is hermetically sealed to the motor housing and is exposed to atmospheric pressure on one side and higher refrigerant pressure on the other. The size of the terminal board is kept to a minimum to minimize the bursting force exerted against the board. However, high voltages require a substantial amount of space between leads to prevent arcing. As a result of the minimally sized board and the required spacing between leads, there usually is not enough room on the board to mount a fourth terminal for the common leads. Moreover, penetrating the board with an additional terminal may weaken the hermetic integrity of the refrigeration system.